Effective Removal of Ibuprofen from Aqueous Solution Using Cationic Surface-Active Agents in Dissolved Air-Flotation Process

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Introduction
Toxic pollutants that are being emitted from domestic and industrial point sources without proper treatment are the major causes of wastewater pollution in urban areas.Te requirement for water is increasing due to the variation in the environment, increase in population, and industrialization [1][2][3][4][5].In the past few decades, the increase of newly identifed compounds by anthropogenic or naturally occurring in surface water has turned into a global issue of escalating environmental concern.Tese compounds are termed organic contaminants popularly known as "emerging contaminants" (ECs) or "contaminants of emerging concern," found even in trace concentrations from parts per trillion to parts per billion and tend to bio accumulate over the period and pose a potential risk to fora and fauna.Tese contaminants include the chemicals found in pesticides, pharmaceuticals (drugs, natural and synthetic hormones), personal care products, fre retardants, plasticizers, food-added substances, and various other compounds released through household and industrial activities [6][7][8][9][10][11]. Amidst these toxic substances, more attention is paid to pharmaceutical and personal care products due to their tremendous creation and wide application.Intense concentrations of a few drug compounds, like ibuprofen (IBU) and sulfamethoxazole (SMX), have come to the consideration of researchers concerning their efects on life in lakes, waterways, and groundwater.Unpredictable removal of unused prescriptions, terminated drugs, and veterinary meds is the major reason why they accumulate in water bodies such as streams, groundwater, surface water, soil, sludge, and sediment samples.IBU is one of the most popular and highly prescribed medicines in the world.It comes under the category of nonsteroidal anti-infammatory drugs that have been used to relieve infammation, fever, and pain involving rheumatoid arthritis, menstrual periods, and migraines [12][13][14][15].Tis has been cataloged in the essential drugs list by the World Health Organization (WHO) in 2010.Tough it varies from ng/L to µg/L, the maximum reported infuent concentration was 60 mg/L.From the recent studies, it was deliberated that there is no limit set for this compound in the environment since it is an over-thecounter drug with more prescription volume and more excretion rate.Ibuprofen has a half-life period of 1.9-2.2hours and is eliminated within 24 hours, so the piling of this compound in surface waters is more frequent.It is recognized in the range of moderate to higher concentration, and the recurrent discharge of these types of efuent without adequate treatment process could end up resulting in prolonged presence in the environment and may beget a negative impact on fora and fauna [16][17][18][19][20]. Tus, a treatment process is needed to fgure out the successful cycles for eliminating these types of noxious pollutants from wastewater to accomplish the critical goal.Tere are many treatment processes available which entail membrane bioreactor, ultraviolet radiation, activated carbon, ozonation, conventional activated sludge, and wetland system, whereas due to their complex characteristics such as high capital cost, membrane fouling, a lot of chemical usage, and difculty in operation, pharma compounds are not efectively removed [6,21,22].Besides, fotation technology is an intriguing method to eradicate wastewater pollutants due to its ease of operation and low footprint.
Flotation technology is a unique physicochemical process that can treat diferent types of wastewaters likely electroplating, mechanical chemical polishing, textiles, fuoride-possessing wastewaters, papermaking, plastics segregation, and recycling.Various types of fotation processes are also enacted in water/wastewater treatment processes such as electrofotation, dissolved air fotation, sorptive fotation, dispersed air fotation, and adsorbing colloid fotation each of which holds distinct characteristics [23].Tere are very few literature studies available on the treatment of nonsteroidal anti-infammatory drugs (NSAIDs) and antibiotics using disparate fotation techniques.Some of them carried the removal of pharmaceutical contaminant in aqueous solution as well as real-time wastewater by utilizing a single type of synthetic-based surfactant [24][25][26].Te relationships between the parameters employed are still unclear in the references that are currently available, and the majority of them entail analysis using a certain kind of collector.So, the main reason for the current study is to scrutinize the best possible ways to remove ibuprofen from wastewater using the dissolved air fotation (DAF) process as this process brings in the separation of targeted contaminants through the generation of gas bubbles with the aid of surface-active agents.Concurrently, three types of cationic surface-active agents as cetyltrimethyl ammonium bromide (CTAB), n-octyltrimethyl ammonium bromide (OTAB), and tetrabutylammonium bromide (TBAB) have been explored and compared to achieve better process efectiveness that was not carried out in other literature studies.Besides, integrated concept is gaining its importance in recent years such that the synergistic efect of best performing surface-active agents has been studied to discover the excellent recovery rates.Moreover, parameters such as initial concentration, surfactant dosage, pressure, pH, and retention time were evaluated in the study.Te kinetics of fotation acting as a speculating tool was made to determine the relationship between contaminant concentration and time.2. Te stock solutions were prepared with distilled water.Operating solutions of IBU were produced by dissolving IBU (100 mg•L −1 ) in distilled water which were subsequently dissolved to a desired concentration.Te components required for the fotation unit were obtained from Dolphin Instruments and Equipments, Chennai.

Experimental Setup.
Te dissolved air fotation setup is a simple batch-type column as given in Figure 1.Te setup consists of a fotation column attached to a compressor (voltage: 220-240 V) and an airfow meter (fow rate: 0-1 L/min) on the right side.Besides, the column was fabricated in acrylic material with a capacity of 850 ml, height of 25 cm, and 10 cm in diameter.Te perforated column was designed to have 25 holes with an air bubble size of 50 µm fxed inside the column.A polyethylene tube with an inner diameter of 0.63 cm and an outer diameter of 0.95 cm was used for the column interconnection along with an air fow meter and a compressor.Te airfow meter fabricated using polymethyl methacrylate (PMMA) comprises a control valve for managing the fow rate having a pressure of 0.1 Mpa.

Selection of Surface-Active Agents.
One of the important factors to achieve successful fotation lies in the selection of surface-active agents.Tis is commonly called a "surfactant," as it decreases the surface tension, thus boosting its wetting 2 International Journal of Chemical Engineering and spreading properties.Based on the categories of nonionic, cationic, and anionic that exist on the market, there are numerous types of surfactants.One such type used in this work is cationic-based.Like other surfactants, it has polar and nonpolar groups possessing a quaternary ammonium unit and various alkyl groups, respectively.Tey carry positively charged ions on their hydrophilic head and can frmly adsorb on the contaminants with negatively charged groups via charge-charge or electrostatic interactions.Te chosen surface-active agents in this experiment work better by providing good solubility and foamability when compared with other types, thereby increasing the fotation efciency better by decreasing CMC formation [27][28][29][30][31][32]. Figure 2 explains the mechanism model of the surface-active agents that tend to be connected with contaminants.

Flotation Procedure.
Te dissolved air fotation is a process that targets the concentrated contaminant based on the variations in physicochemical properties.Each batch of the solution's initial volume (Vo) was prepared in a 500 ml volumetric fask.As IBU is not water-soluble, a very small quantity of ethanol was used to dissolve IBU and made up to the intended volume with distilled water.Initially, the prepared stock solution was frst fed into the fotation column followed by conditioning for 1 minute [33][34][35].Surface-active agents were added gradually at various dosages for solutions containing diferent concentrations.Tey provide proper greater adhesion of particles to bubbles and ions, thus promoting efective transfer to the solution's surface.When fuid phases with varying degrees of polarity and hydrogen bonding, such as air/water interfaces, are in contact with one another, the hydrophobic and hydrophilic moieties get partitioned out.Microbubbles are formed when the air is dissolved at greater pressure in the saturator when the solution present in the fotation column is liberated with atmospheric pressure.Tus, they produce an amicable environment to the froth to be strong enough to protect against scum ruination.Typically, the high-efciency rate of fotation depends on the size of the bubbles as well as the chosen surface-active agents [36][37][38].Bubbles with a size above 150 µm tend to have bubble clusters resulting in poor removal.Hence, in this work, the midsized bubbles (50 µm) are employed as they can provide better lifting power.After achieving a signifcant amount of retention time, the clear liquid is gathered at the bottom, while the froth is ejected at the top.All the experiments were carried out in triplicate.Furthermore, for every fotation experiment, a fresh batch of    [39].Te supernatant obtained after each run of the fotation was enacted for the quantitative analysis using UV-VIS.As the pharmaceutical compounds are light sensitive, the collected samples for analysis were wrapped with the aluminium foil as well and the analysis was carried out immediately after each trial to avoid degradation.At the end of each trial session, utilised laboratory glassware and the fotation column were consistently rinsed with distilled water in order to enhance better results.

GC-MS Assay.
Trough GC-MS, qualitative analysis was made to fnd out the metabolites if any formed during the process.Te Clarus 680 GC was used in the analysis of pharmaceutical compound degradation with a fused silica column, packed with Elite-5MS (5% biphenyl 95% dimethylpolysiloxane, 30 m × 0.25 mm ID × 250 μm df ), and the elements were separated using helium as a carrier gas with a constant fow of 1 ml/min.Te injector temperature was set at 260 °C during the chromatographic run.Te 1 μL of extract sample was injected into the instrument accompanying the oven temperature as follows: 60 °C (2 min); followed by 300 °C at the rate of 10 °C min −1 ; and 300 °C, where it was held for 6 min.Te total run time was up to 30 minutes.

Dynamic Surface Tension Measurements.
Te surface tension was measured using a DY-300, Kyowa Interface Science Co., Ltd using a movable plate method at a temperature of 27 °C.Te platinum plate and glass containers were thoroughly washed with deionized water.Before carrying each experiment, the plate was fame dried.Surface tension was taken to be at equilibrium, when the average standard deviations of the fve consecutive values did not exceed 0.10 mN/m.

Results and Discussion
3.1.Efect of Surface-Active Agents.Aforementioned, this is one of the paramount operating parameters for successful fotation.Figure 3 shows variations in IBU removal using diferent surface-active agents keeping other parameters constant.Te dosages that were varied to attain better removal lie from 0.2 g to 1.4 g.Te maximum removal efciency concerning CTAB was about 96.09%, TBAB had 62.36%, and 89.6% was for OTAB.When compared with the three surface-active agents, a better removal rate was observed at 0.6 g of CTAB as it possesses 16 carbons, whereas the target contaminant has 13 carbons.So, the better removal efciency was achieved in 15 min which denotes that the IBU had the highest afnity to produce micelles with CTAB.Besides, IBU removal was good with the other two surfactants on the dosages of 1.0 g and 1.2 g for OTAB and TBAB, respectively.Since TBAB has lesser foaming ability compared with CTAB, lesser dosages do not yield the expected results.Te performance of OTAB with IBU was fair enough as OTAB has 8 carbons.In addition to that, the results with TBAB were very poor as it is a branched alkyl chain compound, but the other two surface-active agents were linear alkyl chain compounds.It can be inferred that the geometric shape of the surfactants tends to be one of the deciding factors to procure a better removal rate [40].Te variations of dynamic surface tension with three surfactants at diferent dosages in fotation are shown in Figure 4. Te same behaviour was observed for all the surfactants that the surface tension got reduced with the increase in the surfactant dosage, and this may be due to the fact that the adsorption of surfactant monomers at the air-water interface and tension remains constant after attaining the critical micelle concentration.Te surface tension of double distilled water was about 71.85 mN/m.It was found that the value of surface tension for CTAB decreased to 28.52 mN/m which showed the greatest afnity, TBAB of 47.2 mN/m, and OTAB of about 31.06 mN/m.International Journal of Chemical Engineering

Efect of pH and Concentration.
It is crucial to comprehend the impact of pH on the removal process since changes in solution pH afect the IBU speciation.Ibuprofen has a pKa value of 5.2, and they occur in neutral form at pH < pKa.Tis compound occurs as neutral species at coexists at neutral and anionic at pH≈pKa and occurs as anion species at pH > pKa.Ibuprofen appears in neutral form at pH 4 which is below pKa and in anionic form above pKa at pH 7. Figure 5 shows the efect of pH on the removal efciency of IBU by fotation.Te experiment was validated at pH 4 (acidic), around pKa of pH 5.3, and pH 7 (neutral) to scrutinize the sorption of ibuprofen with anionic, neutral, and neutral/anionic species onto the sediment [41].Ibuprofen is a weak acid, and its removal was favorable at pH 4 and unfavourable at pH 7. Acidic pharma compounds are neutral molecules, that is, the pH which is below pKa, interacting with the column surface via nonelectrostatic interaction by entailing hydrogen bonds.In this experiment, at pH 4 (lesser than pKa), ibuprofen neutral was sorbed via nonelectrostatic interaction with the settling surface and this follows [42].
Te rate of removal decreases with the increase in pH as there occurs deprotonation of surface-active agents which made it very complex to act on ibuprofen, obstructing the formation of agglomerates [43][44][45].Te concentration of IBU was varied from 25 mg L −1 to 100 mg L −1 .Te separation efciency concerning the initial concentration is shown in Figure 6.As the concentration increases, the separation rate decreases.Stable froths were formed when increasing the concentration from 25 mg L −1 to 50 mg L −1 .Te maximum removal rate of about 96.51% was observed at 50 mg L −1 with CTAB, 74.11% for OTAB, and 64.44% for TBAB.Te rate of particle-air bubble collision and detaining probability lead the way to the production of charged froths at 50 mg L −1 , balanced by fne particles.Moreover, there was a sufcient contact between the contaminant and surface-active pairing.Te TBAB results were not as expected since it possesses a shorter chain length and exhibited lesser binding efciency.Besides, attaining the critical point, the rate of removal decreases at 75 mg L −1 and 100 mg L −1 for CTAB and OTAB.Tis is due to the following factors: (1) formation of complexes with other ions, (2) attachment site competition, (3) saturation of air bubbles, and (4) the size of the bubbles getting reduced [46,47].International Journal of Chemical Engineering

Efect of Contact Time.
Te efect of contact time concerning the fotation removal rate has been discussed after fnding out the system composition.Te fotation time was varied from 5 to 60 min, and the samples were taken at every 5 min, which is shown in Figure 7.As mentioned, the removal efciency of the contaminant with CTAB of about 96.09% was achieved in 15 min.Te removal rates with OTAB and TBAB were obtained at 30 min and 45 min, respectively.Certainly, the fotation mechanism involves three phases: (1) true fotation, that is, the particle-bubble attachment, (2) entrainment, and (3) drain.Te frst two processes appear in the suspension zone, while the last one takes place in the froth zone.In the froth zone, liquid flm thinning happens due to free water gravitational drain, by which the interfacial tension infuences the frmness of liquid flm between the solid-liquid interface [48][49][50].From the procured results, it can be seen that the rate of removal escalates with the increase in fotation time.It was observed that, for CTAB, there was a rapid removal within 15 minutes.Tis is because the ideal fotation time made the target contaminant thoroughly contact with the formed micelles, and there occurred a high chance of particle bubbles colliding (target object and bubbles) and bubble surface loading increased [40].However, the prolonged fotation time made the interface between the two phases impaired, and this applies to every surface-active agent.

Efect of Flow Rate and
Pressure.Te fotation rates were determined as a function of fow rates that were varied from 0.25 L/min to 1.0 L/min by keeping other parameters constant.It was observed that the bubble agglomeration could not be seen on the fow rate of 0.25 L/min.Te fotation recovery rates increased to the maximum and decreased with the increase in the fow rate at 0.75 and 1 L/min, and it was due to the formation of bubble coalescence [51].So, the optimized fow rate was found at 0.5 L/min which is shown in Figure 8.
Te fotation study was conducted with a pressure variation from 5 psig to 30 psig (pound per square inch), and pressure was varied for every 5 psig.Te size of the bubbles is afected due to the diferences in the pressure given across the system.It was reported that the bubble size that is produced below the pressure of 4-6 atmospheres is generally reported to be in the size between 10 and 100 µm keeping the average size of 40 µm [52].Accordingly, the chosen bubble size in this experiment is 50 µm.From the procured results, the maximum removal was perceived at 20 psig with CTAB and 25 psig for OTAB and TBAB.It was apparent that the removal efciency escalates with the increase in pressure which implies that it can bear a greater number of contaminants to the air-water interface and improves the removal rate of IBU [53].However, it was identifed that there is a critical point, after which the removal rate decreases as seen in Figure 9, exhibiting the removal rate of IBU concerning pressure.After 20 psig, the rate of removal declines due to bubble coalescence ensuing in the dissemination of some of the contaminant-collector precipitates that get back into the bulk solution.As a consequence, there is an inadequate contact between the surface-active agent and the contaminant [54].

Synergistic Efect of Surface-Active
Agents.CTAB and OTAB were combined to fnd out the best possible removal rate of contaminants and the synergistic efect of surfactants.It was observed that the cationic/anionic (IBU) systems indicated improved synergism and formation of mixed micelle.It was observed that there was a complete removal by fotation in 25 min with 0.4 g and a pressure of 20 psig.6 International Journal of Chemical Engineering Tis synergistic efect augments surfactant-based occurrence to be better in forming a good foaming efect and tends to have lower CMC.As both of them are linear alkyl chain compounds, the surface-activeness was more than the application of individual surfactants [55][56][57][58][59]. Te elimination of contaminants was marginal at higher dosages of the combined surfactants, such that the optimal dosages must be employed.Te removal percentage of ibuprofen with the integrated surface-active agents is shown in Figures 10  and 11.

Gas Chromatography Mass-Spectrometry Analysis (GC-MS).
As fotation involves phase change but to identify the metabolites formed and understand its level of toxicity, GC-MS analysis was performed.Full scan mass spectra of Ibuprofen in Figure 12 were taken as a precursor which had a m/z of 205.11 with the retention time of 4 min.Analytical GC/MS outcomes of solutions after the interaction between ibuprofen and the surface-active agents in Figure 13 show the relative intensity of formed metabolites in the supernatant.A very few literature studies were available on the analysis of GC-MS of pharmaceutical compounds with respect to cationic surfactants.From the observed chromatographic profle for ibuprofen in accordance with TBAB, the existence of TBAB was confrmed by its mass spectrum through the fragments formed of long-chain aldehydes such as tri-decanal (with high intensity), DL-3-amino butyric acid, and N-hexadecenoic acid at a retention time of 1.    derivatives were not visible as they occurred in a trace amount and the formed intermediates due to surface-active agents were high [60][61][62][63].
In the case of OTAB, a high intensity of 2-octanol, 2,6dimethyl was visible with a retention time of 1.6 minutes.Also, eicosanoic acid with a retention time of 17.83 and oleic acid with 19.67 minutes were observed.Te formed compounds were a residue of ibuprofen degradation during the process.In this sense, there occurred complete degradation of the pharma compound, while some derivatives were found during the interaction of IBU with a CTAB, a surface-active agent.Te formed derivatives were 14-pentadecanoic acid with high intensity, tetra decanoic acid, dodecanoic acid, and methyl 9eicosenoate [64,65].All tested derivatives possessing high intensity are less toxic than the parenting compounds.

Flotation Kinetics
Flotation kinetics, which is paramount in illuminating the mechanism of the process and acting as predictive tools in the use of fotation technology, is employed to scrutinize the variation of concentration over time.Te present work investigates the fotation rate in accordance with the most extensively used pseudo-frst order (1)and second order kinetics (2) Also, concerning the diferent surface-active agents employed in the research, the fotation kinetic models have also been employed to CTAB, OTAB, and TBAB to predict the best possible removal mechanism.Te following equations illustrate the kinetic models as For assessing fotation performance, the fotation rate constant (K) and maximum theoretical recovery (R∞) are the helpful indicators, while t represents the fotation time [66,67].
Te graphs are allotted, and the R 2 values are taken.Te degree of accuracy of linear ft was quantifed by a higher R 2 value.Even the retention time of fotation is extended indefnitely, and it is unlikely that the recovery of hard foating particles with weak hydrophobicity will be 100%.Figure 14 shows the plots of experimental data ftted by the two kinetic models occurred at diferent fotation time intervals, pertaining to the actual fotation yields [54,[68][69][70].Te parameters of the kinetic models are summarized in Table 3.
In comparing with the pseudo-frst order kinetic model, the fotation kinetics of surface-active agents with respect to the contaminant could be well described by pseudo-second order kinetics.In accordance with the variance between the estimated and the experimental outcomes, holds the R 2 value of CTAB, OTAB, and TBAB as 0.9966, 0.9949, and 0.9912, respectively, which follows [71].Terefore, it may be said that the pseudo-second-order kinetic model suits well to describe the fotation behaviour of the contaminant eradication.

Conclusion
Te study focused on the removal of an emerging contaminant from an aqueous solution using fotation technology.A full-pressurised dissolved air fotation process was employed, as an environmentally friendly and sustainable approach.Te diferent experimental conditions have been carried out with the optimization of infuencing parameters such as surfactant dosage, pH, retention time, pressure, fow rate, and initial concentration.From the analysis obtained and discussions that were presented in the preceding sections, concluding remarks may be drawn as follows: this technology is well known for its minimal generation of sludge and high throughput despite its difculty in removing scum [47,72].
Comparison of various cationic surface-active agents such as cetyl trimethyl ammonium bromide (CTAB), tetrabutyl ammonium bromide (TBAB), and octyltrimethyl ammonium bromide (OTAB) has been employed.Te scrutinization of the study was made to profoundly understand the behaviour, acclimatization, and drawbacks of those collectors on the efective removal of the pharmaceutical compound.
(1) Among the three surface-active agents used, it was observed that the CTAB contributed efectively to the removal of ibuprofen through dissolved air fotation.Te rate of removal of about 96.09% was achieved at 15 min for CTAB, while TBAB had 62.36% at 45 min, and 89.6% was obtained for OTAB at 30 min.(2) Moreover, the structures of the cationic collectors possess a greater impact on the removal of the contaminant.It can be inferred that the performance of the branched alkyl chain compound, TBAB, was very poor.However, the other two cationic collectors with a similar structure in the alkyl chain are found to be the ideal collectors for the fotation process providing good foamability.CTAB and OTAB were combined to fnd out the best possible removal rate of contaminants.Te synergistic efect augments surfactant-based occurrence to be better in forming a good foaming efect and tends to have lower critical micelle concentration (CMC).(3) On the other hand, the formed metabolites that had been identifed using gas chromatography-mass spectrometry are found to be less toxic than the parenting compounds.Te efect of cationic collectors' performance of ibuprofen was also studied from the perspective of fotation kinetic models.Nevertheless, two kinetic models have been made, and we found that the pseudo-second-order kinetic model possesses a greater ftting accuracy (R 2 > 0.998), and the ftted curve more precisely depicted how the concentrate yield difered over time.As an outcome, it can be concluded that the pseudo-second-order kinetic model is more suited to represent the contaminant's foating behaviour.(4) Furthermore, the dissolved air fotation process possesses advantages of being efcient and simple, having the ability to remove all types of pollutants, having less retention time, and causes the generation of sludge.But the main disadvantage of the process lies in the difculty of removing scum, which requires a necessary attention.Additionally, given the experimental conditions tested, the collector used in this process produces improved separation efciency and might be successfully applied [73].

Figure 2 :
Figure 2: Micelle aggregation of the cationic surfactant with a hydrophobic particle.

Table 2 :
Te chemical properties of the surface-active agents used for IBU removal.

Table 1 :
Te physicochemical properties of ibuprofen.

Table 3 :
Comparison of pseudo-frst and second-order kinetic parameters for pharma compound under various surface-active agents.